It is generally known that dry-type transformers are used in power distribution systems to adapt the voltage levels of different system components. These have a power range of, for example, 100 kVA to some 10 MVA and are provided for rated voltages between, for example, a few kV to 110 kV. In contrast to oil-filled transformers, a liquid insulation medium is dispensed with here; rather, for example, the windings are surrounded by a solid insulation material, such as a fiber roving which is impregnated with resin and subsequently cured. In the case of a three-phase transformer, three windings of this type are then arranged around each limb of a transformer core. To dissipate the heat loss which occurs during operation, cooling ducts through the windings are necessary, for example, to a greater extent in the case of a hot working environment.
For certain applications, it is necessary to encapsulate dry-type transformers, to arrange them inside a closed housing. This can be the case, for example, for dry-type transformers which are arranged inside a very cold working environment or else on a ship or an oil-drilling platform and against the effects of salt-containing sea water. However, an encapsulation can occasionally be necessary merely because of the mechanical protective effect thereof. For example, in the event that an encapsulated transformer is subjected to frequent shutdowns depending on use, condensation can form inside the transformer housing since the transformer then cools down owing to the lack of heating effect of the winding losses. Even during permanent operation of an encapsulated transformer, condensation can be formed if the transformer is positioned in such a cold working environment that the winding losses do not lead to sufficient heating of the transformer or the housing thereof, with the result that the dew-point temperature is not exceeded. In this case, in the event of relatively long shutdowns, condensation collects on the interior surface of the housing or also on the surface of the windings.
In order to avoid condensation appearing inside a transformer housing, it is therefore usual, depending on requirements, to integrate space heaters in the housing, for example radiators. The radiators are often arranged, owing to a simplified arrangement, on either side of the transformer core on the base of the housing and have a heating region which is directed upward. As a result of this, the regions on either side of the transformer core are heated and the formation of condensation is avoided there. However, the transformer windings and the housing region thereabove are not directly covered by the heating region and so condensation can form there which can drip in a disadvantageous manner, for example from the upper housing region, onto the transformer windings located below.